Conventionally, in a structure for fixedly connecting an impeller and a back yoke of an outer rotor centrifugal blowing fan, a back yoke made from metal was joined to an accommodation being formed integrally with an inner circumferential portion of an impeller made from resin by means of inserting and/or thermal adhesion. However, the expansion of applications has required that such centrifugal blowing fan is in use under severe environment, whereat the problem arises that the accommodation is damaged due to the difference of thermal shrinkage between the accommodation made from resin and the back yoke made from metal. Further, forced draft cooling under a fitted space-saying condition and consequently miniaturization of the centrifugal blowing fan are required.
To solve this problem, known are examples in which a structure is adopted where an impeller and a back yoke are connected instead of covering an outer circumference of the back yoke with an accommodation made from (See, for example, Patent Literature 1, Patent Literature 2 and Patent Literature 3).
Patent Literature 1 discloses a structure where a ring portion made from resin is insert-molded into an opening or an outer surface of a back yoke made from metal, and further an impeller is ultrasonic-welded to the ring portion to fixedly connect the impeller and the back yoke.
Patent Literature 2 discloses a structure where a back yoke and an impeller are integrally formed from one steel plate.
Patent Literature 3 discloses a structure where a flange portion is provided at an opening of a back yoke, and blades of an impeller made from metal are mounted on the flange portion. In this structure, the blades are made from metal, and those blades need to be mounted on the flange portion one by one.
Since, however, the technique according to Patent Literature 1 discloses a structure where the ring portion made from resin is insert-molded into the opening or the outer surface of the back yoke made from metal, followed by ultrasonic-welding the impeller to the ring portion, the technique would require a lot of man-hours, resulting in an increase in costs. That is; the back yoke formed by press working is set in a mold for insert-molding to mold the ring portion, and subsequently the impeller which has been made by a different process is integrated by ultrasonic-welding and the like with a member obtained by connecting the back yoke and the ring portion. In this case, it is needed to align the central axis of the impeller and the central axis of the back yoke, but it is not so easy to align the central axes since the impeller and the ring portion are different members. Therefore, even it for example, cross-sectional shapes of impeller blades are engraved on the ring portion, a work to place each of the blades in the engraved portions is still needed. This work requires a lot of man-hours much more compared to the conventional method, wherefore the increase in costs remains as a problem. Further, a location whereat the ring portion is connected with the back yoke is in the vicinity of the opening and/or the outer surface of the back yoke, which means that this location is near by a source of heat. Therefore, this involves the problem that the location would be subject to influence by heat.
The technique according to Patent Literature 2 discloses a structure where the back yoke and the impeller are integrally formed from one steel plate. However, this structure involves the problem that it is difficult to realize such structure in case of an impeller with a certain height or an impeller having a lot of blades.
The technique according to Patent Literature 3 discloses a structure where the flange portion is provided at the opening of the back yoke, and blades of the impeller made from metal are mounted on the flange portion. In this structure, the blades are made from metal, and the blades have to be mounted on the flange portion one by one. Therefore, this structure involves the problem that a lot of man-hours are required, resulting in an increase in costs.